Win32: avoid using CFLAGS, just add define instead.

With CFLAGS set as in 7da71a7b141a, OpenSSL compilation drops various
non-important compiler options.  To avoid this, a define is added
instead - OpenSSL is smart enough to recognize -D... in Configure
arguments.

misc/GNUmakefile

@@ -82,8 +82,7 @@ win32:
 		--with-mail						\
 		--with-stream						\
 		--with-openssl=$(OBJS)/lib/$(OPENSSL)			\
-		--with-openssl-opt="no-asm no-tests			\
-			CFLAGS=-D_WIN32_WINNT=0x0501"			\
+		--with-openssl-opt="no-asm no-tests -D_WIN32_WINNT=0x0501" \
 		--with-http_ssl_module					\
 		--with-mail_ssl_module					\
 		--with-stream_ssl_module